Homework Assignment No. 8Problem 9.35•A flow of 2 kg/s saturated vapor R-22 at 500 kPa isheated at constant pressure to 60◦C. The heat issupplied by a heat pump that receives heat from theambient at 300 K and work input. Assume the process isreversible and find the rate of work input.SolutionThe control volume is the heat exchanger. From the conservation of mass,˙mi= ˙me= ˙m. From theFirst Law of Thermodynamics,0 =˙QH+ ˙mhi−˙mhesince the change in kinetic and potential energy across the heat exchanger is negligible. From TableB.4.1, for saturated R-22 vapor,hg= 249.95 kJ/kg atp= 497.6 kPaandhg= 251.73 kJ/kg atp= 583.8 kPa650:351 ThermodynamicsPage 1

Homework Assignment No. 8Problem 9.36•A reversible steady-state device receives a flow of 1 kg/sair at 400 K and 450 kPa, and the air leaves at 600 Kand 100 kPa. Heat transfer of 800 kW is added from a1000 K reservoir, 100 kW is rejected at 350 K, andsome heat transfer takes place at 500 K. Find the heattransferred at 500 K and the rate of work produced.SolutionThe control volume is the device. The conservation of mass is˙mi= ˙me= ˙mThe First Law of Thermodynamics is0 =˙Q1−˙Q2+˙Q3−˙W+ ˙mhi−˙mheNote sign for˙Q2which is assumed positive for heatremovedThe Second Law of Thermodynamics is0 = ˙msi−˙mse+summationdisplay˙QT+˙Sgen650:351 ThermodynamicsPage 4

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